Printed matter is ubiquitous in modern society. There are manifold methods of producing printed documents, ranging from large-scale offset printers to small scale personal inkjet printers. Each printed document, regardless of printing mechanism, shares the characteristic that light reflected from that document depends on properties of a medium on which the document has been printed. That is, both colour and reproduction quality of the document depends sensitively on surface properties of the medium, such as bulk physical and chemical properties of the medium. The colour and reproduction quality of the document also depends on optical properties of the medium.
In terms of effect of the optical properties of the medium, light that falls on the document and is reflected towards a user can have taken many paths. In the instance that the medium is paper, some of the light will have been directly reflected off the surface of the paper, and some will have entered the paper and been scattered once or many times. Some of the light will have entered the paper and been reflected off the back surface of the paper. Other light will be absorbed, either by the ink or toner that has been printed on the paper, or by the paper itself.
The diversity of different optical paths within the paper leads to an effect known as optical dot gain (ODG). Optical dot gain may be understood by considering a single line of black toner printed onto the surface of a page. Light that falls onto the toner is mostly absorbed, so the toner itself appears black. Light that falls on the paper surface that is directly next to the edge of the line will ether be reflected off the surface directly, or will enter the bulk of the paper and be scattered there. If the light enters the bulk of the paper, the light has a roughly 50% chance of being scattered under the line of toner, there to be absorbed. This means that only 50% of the light that enters the paper at the edge of the line can escape. For a point further away from the line of toner, a much smaller portion will scatter under the line of toner. This means that areas further from the line of toner will appear brighter than the areas closer to the toner.
In this way, if a dense set of lines is printed on a page that covers exactly 50% of the page, then far less than 50% of the incident illumination will be reflected from the page. Light that falls on the lines will be absorbed, and light that falls between the lines has some probability of entering the paper and scattering under the lines to be absorbed. The actual fraction of the light that scatters and is absorbed is a property of the paper (or other medium) and varies from paper type to paper type, and may also vary slightly within a given paper type or even on a given sheet depending on the formation properties of the light.
Given that optical dot gain changes in amount of reflected light from printed paper, optical dot gain plays an important role in visual characteristics of printed mediums. In particular, if accurate colour reproduction is required, then the effect of the optical dot gain for a particular medium needs to be accounted for. Typically, optical dot gain for a particular medium is accounted for by producing a colour calibration profile, such as an International Color Consortium (ICC) profile, for a given printer and print medium combination. The colour calibration profile is produced essentially by printing a large number of colour patches and experimentally determining what the effect of optical dot gain (and other processes) is for a printer and print medium combination. Such a process has the draw-back that it is necessary to produce a colour profile for every print media to be used with a given printer, and producing colour profiles is an expensive process both in terms of time and of equipment.
Another method for dealing with the effects of optical dot gain is to model the effect of optical dot gain for a given media. If this is possible, then it may be possible to produce a profile for a printer that is largely independent of the type of media that is being printed on. Optical dot gain is not the only important factor to understand to produce such a system. For example, gloss or surface roughness is another important property. However, if optical dot gain can be modelled then optical dot gain may be compensated for. In order to make use of effective models of optical dot gain, then the properties that give rise to optical dot gain ODG must be measured.
Optical dot gain also affects the sharpness of shapes and characters on printed documents. This sharpness is reflected in measurements of printer resolution, which measure the ability of a printing system to reproduce fine details. Optical dot gain changes these measurements of printer resolution by changing the amount of light that is reflected between and around lines printed on paper. So, for a fixed printing system, different paper types can lead to different measurements of resolution for the printing system, independent of the actual mechanism of the printing system. This is clearly an undesirable property for measurement of a printing system which should be independent of the paper type.
While printer resolution is one of the important printer quality metrics, there has not been any industry standard resolution metrics for digital printers. In the ISO/IEC 13660 (2001) image quality standard, no standardised measurements are related to printer resolutions. In printer resolution measurement, a common method of evaluating one dimensional resolution of the printer is a Contrast Transfer Function (CTF) measurement. The process of CTF measurement includes printing CTF patterns, which consists of black and white bar charts of varying spatial frequencies, imaging the CTF patterns and measuring the contrasts at the different spatial frequencies.
For a given imaging device, the CTF measured is actually the result of physical dot gain and optical dot gain combined. The physical dot gain is related to the printer and the printing process, while the optical dot gain is due to the optical scattering of light within the print media in the reflective imaging process. In order to precisely and repeatedly evaluate the resolution of a printer, it is desired to quantify the physical dot gain effect and the optical dot gain effect separately.
Typically, CTF measurement is performed by measuring the minimum (Dmin) and maximum (Dmax) optical densities of the black and white lines on the CTF patterns. The contrast, (Dmax−Dmin)/(Dmax+Dmin), at different spatial frequencies is commonly used as a resolution metric. But this resolution metric is by no means the only resolution metric of printers. Another known method is to use (Dmax−Dmin) as the resolution metric, as avoiding division by (Dmax+Dmin) is considered to be more representative of human perception.
Measurements of optical dot gain properties of different paper types have been performed for quite some time. There are a number of different conventional methods used to measure optical dot gain. In one such method, an image of an edge is projected on to a sheet of paper and the sharpness of the edge reflected off the paper is measured. In another method of measuring optical dot gain a laser spot is focussed onto a piece of paper and the distribution of light around the spot is measured using high-resolution imaging or a microdensitometer. In still another method, an image with patches of lines of different spacings may be printed on a medium. The printed medium may then be scanned across at high resolution to determine the profile of reflected light between the lines in order to infer parameters of some functional parameterisation of light scattering probability. For example, the light reflected from the medium may be measured using a one dimensional sensor scanned across the test pattern.
In still another method of measuring optical dot gain, images with patches of lines with different spacing are produced on stripping film. The film is placed in contact with a medium and then the medium is scanned at high resolution to infer parameters of some functional parameterisation of light scattering probability. Finally, in another method of measuring optical dot gain, an image of a patch of lines is projected onto a sheet of paper. The paper is then imaged through the patch of lines at high resolution to infer the parameters of some functional parameterisation of optical scattering probability.
The above optical dot gain measurement methods that directly measure the optical dot gain properties of the medium require expensive measurement apparatus, high resolution imaging. The above optical dot gain methods are also sensitive to noise and have poor reproducibility. The methods that infer the optical dot gain properties require high resolution imaging, and depend on the functional parameterisation of the optical dot gain.
The above methods of measuring printer resolution often give different results based on subtle properties of a print medium on which the test target is printed. Thus, a need clearly exists for a more efficient method of evaluating printer resolution which is largely independent of the paper being used.